Relaxational change of the phase transition character in ferroelectric-semiconductor

Abstract

The effect of time evolution of second-order ferroelectric phase transition (PT), which is close to the Lifshitz point and the tricritical point in the phase diagram, to the sequential second-order and firstorder transitions has been found. As the data on time variations of the temperature dependence of dielectric constant for Sn2P2S6 ferroelectric-semiconductor show, the crystal exposure at fixed temperature in the paraphase in the vicinity of the second-order PT results in the appearance of an intermediate (incommensurate, hypothetically) state. Sample lighting benefits the formation of such state, whereas a constant electric field directed along the spontaneous polarization axis reduces its temperature range. Experimental data are described qualitatively by mean-field approximation within a framework of a model, that suggests the linear dependence of thermodynamic potential coefficients of the uniaxial proper ferroelectric with a single direction of modulation in the incommensurate phase vs the charge ...